Democratic presidential frontrunner Hillary Clinton sparred in a heated confrontation with an environmental activist on Thursday over her campaign contributions from fossil fuel companies, accusing rival Bernie Sanders' campaign of "lying" about the issue.

The encounter came as Clinton was greeting voters following a rally at the State University of New York-Purchase.

"I do not — I have money from people who work for fossil fuel companies," she responded, distinguishing between contributions from energy companies themselves and those employed by them. "I am so sick of the Sanders campaign lying about me. I'm sick of it."

The background: The exchange came as Greenpeace and the Sanders campaign have hammered Clinton over contributions linked to fossil fuel companies.

A Greenpeace analysis found that Clinton's campaign and allied super PACs have received upward of $4.5 million from industry lobbyists, bundlers and other large donors linked to fossil fuel concerns.

That count excluded contributions from energy executives and rank-and-file employees. According to the Center for Responsive Politics, Clinton's 2016 campaign has received $307,561 from people working in the industry. That placed Clinton behind Sen. Ted Cruz of Texas, who's raked in more than $1 million; former Florida Gov. Jeb Bush, who hauled in about $500,000; and Sen. Marco Rubio of Florida, who collected more than $350,000.

But Sanders has also received contributions from industry employees, taking in $53,760.

Notably, the Vermont senator's campaign has opted to focus instead on the contributions Clinton's campaign and super PACs have received from industry lobbyists.

"The truth is that Secretary Clinton has relied heavily on funds from lobbyists working for the oil, gas and coal industry," Sanders spokesman Michael Briggs said in a statement Thursday. "If Secretary Clinton wants to discuss this and other important issues she should stop stalling and agree to a debate in New York before the April 19 primary election."

Though Clinton won the endorsement of the League of Conservation Voters and has laid out an ambitious clean-energy agenda, her record on climate issues has been a matter of contention among many activists.

As secretary of state, Clinton said she was "inclined" to approve the Keystone XL pipeline project, which would have transported oil from Canada's tar sands to the Gulf of Mexico, though she now opposes the pipeline's construction.

Moreover, as an extensive Mother Jonesinvestigation documented, the Clinton State Department aggressively promoted natural gas fracking globally. Sanders has called for an end to fracking, while Clinton supports it, with regulation.